full complement of hydrogen atoms are apt to be solids at the ordinary
temperature of the air. It is common to call the former "oils" and the
latter "fats," but that implies too great a dissimilarity, for the
distinction depends on whether we are living in the tropics or the
arctic. It is better, therefore, to lump them all together and call
them "soft fats" and "hard fats," respectively.

Fats of the third order, the stearic group, are called "saturated"
because they have taken up all the hydrogen they can hold. Fats of the
other two groups are called "unsaturated." The first, which have the
least hydrogen, are the most eager for more. If hydrogen is not handy
they will take up other things, for instance oxygen. Linseed oil, which
consists largely, as the name implies, of linoleic acid, will absorb
oxygen on exposure to the air and become hard. That is why it is used in
painting. Such oils are called "drying" oils, although the hardening
process is not really drying, since they contain no water, but is
oxidation. The "semi-drying oils," those that will harden somewhat on
exposure to the air, include the oils of cottonseed, corn, sesame, soy
bean and castor bean. Olive oil and peanut oil are "non-drying" and
contain oleic compounds (olein). The hard fats, such as stearin,
palmitin and margarin, are mostly of animal origin, tallow and lard,
though coconut and palm oil contain a large proportion of such saturated
compounds.

Though the chemist talks of the fatty "acids," nobody else would call
them so because they are not sour. But they do behave like the acids in
forming salts with bases. The alkali salts of the fatty acids are known
to us as soaps. In the natural fats they exist not as free acids but as
salts of an organic base, glycerin, as I explained in a previous
chapter. The natural fats and oils consist of complex mixtures of the